Hello julie-a,

The IRS has Rules for determining who is entitled to claim a dependent.  You need to make sure your dependent meets the IRS requirements. Generally, the IRS requires that the child is under the age of 19 (or under 24 if a full-time student), lives with you for more than half the year, and does not provide more than half of their own financial support.  Living away at school would be the same as living with you if they come home when not in school.

You'll need to paper-file your return, as the IRS will reject your attempts to e-file.  The IRS will then notify both of you who claimed the dependent to determine who is entitled to claim the dependent using the Tiebreaker Rules.

From the IRS, the Tiebreaker Rules, these appear to meet your situation:

• If the parents don’t file a joint return together but both parents claim the child as a qualifying child, the IRS will treat the child as the qualifying child of the parent with whom the child lived for the longer period of time during the year. If the child lived with each parent for the same amount of time, the IRS will treat the child as the qualifying child of the parent who had the higher adjusted gross income (AGI) for the year.
• If no parent can claim the child as a qualifying child, the child is treated as the qualifying child of the person who had the highest AGI for the year.
• If a parent can claim the child as a qualifying child but no parent claims the child, the child is treated as the qualifying child of the person who had the highest AGI for the year, but only if that person’s AGI is higher than the highest AGI of any of the child’s parents who can claim the child. If the child’s parents file a joint return with each other, this rule can be applied by dividing the parents’ combined AGI equally between the parents.

Helpful links:

Rules for Claiming Dependents on Taxes

Divorced with 2 dependents

What if somebody already claimed my dependent?

Qualifying Child of More Than One Person